1. Field of the Invention
This invention relates to a method of treating the surface of an iron alloy or similar material in a fluidized bed furnace to form a carbide or nitride layer thereon and an apparatus for carrying out the method.
2. Description of the Prior Art
A furnace having a bed of alumina powder into which gas, such as air or argon, is blown to fluidize it, is used as a heat medium for the heat treatment of steel. This heat medium has a uniform temperature distribution and can transfer heat quickly. Therefore, the furnace can rapidly and uniformly heat the material to be treated.
A known method employs a fluidized bed furnace to form a diffused metal carbide or nitride layer on the surface of an iron al3oy or similar material. Gas of a halide of an element for forming a carbide or nitride, hydrogen gas, and nitrogen or hydrocarbon gas are, for example, introduced into alumina powder in a furnace to form a fluidized bed. The material to be treated is placed in the fluidized bed and heated. The halide gas is decomposed on the material to form a carbide or nitride layer on its surface. This method, however, has a number of disadvantages. Separate apparatus is required for generating the halide gas. The hydrogen gas, which is used as a carrier gas, involves the danger of explosion. The furnace cannot be opened to enable removal of the material immediately after it has been treated. No rapid quenching is, therefore, possible when the material is cooled. If its hardening is required, it calls for quenching treatment.
FIG. 3 shows an apparatus which is used to carry out method developed by the inventors of this invention in order to solve the problems as hereinabove pointed out. A mixture a of the powder of a treating agent and the powder of a refractory material, such as alumina, is put in a fluidized bed furnace b. The treating agent is composed of a metallic element for forming a carbide or nitride or an alloy thereof, and a promotor, such as ammonium halide. A fluidizing gas, such as argon or nitrogen, is introduced into the furnace b through a gas supply passage c to form a fluidized bed. The material d to be treated is placed in the fluidized bed and the furnace b is heated by a heater e, whereby a carbide or nitride layer is formed on the surface of the material d. This method is safe, as it does not use any hydrogen or halogen vapor.
This method is, however, not free from any problem. If the material to be treated has a complicated shape or consists of a large number of small articles which are closely with one another in the treating agent, the powder of the treating agent, which is highly reactive with the material to be treated, is likely to adhere to the surface thereof. The adhering powder causes the seizing or wear of the material if it is, for example, a stainless steel strip forming die or an extrusion die for cold forging. Therefore, it is necessary to remove the adhering powder. When the treating agent has decreased its power of forming a carbide or nitride layer, it is necessary to change the mixed powder a as a whole.